1. Field of Invention
The present invention relates to antimicrobial susceptibility testing using antimicrobial agent diffusion in a growth medium, including for example agar-based media. More specifically, the invention relates to an improved assay enabling a result with less required time and/or difficulty.
2. Discussion of Most Relevant Prior Art
The disclosure of U.S. Pat. No. 7,262,021, issued Aug. 28, 2007 to the inventor, describes diagnostic apparatus and methods usable in performing direct antimicrobial susceptibility testing. In addition, the disclosure teaches examples of how such testing can run concurrently with isolation, quantitative analysis and presumptive identification of an organism under test. The assay of the prior disclosure provides a way to obtain next-morning antimicrobial susceptibility results. This gives the health care provider the option to prescribe an antimicrobial agent based on evidence instead of merely making assumptions about an infection organism's susceptibility to a particular antimicrobial agent prescribed.
The short delay in prescribing until after the results of the testing have been obtained enables a better choice of antibiotic to treat the patient. It generally allows for a treatment using a narrower spectrum, less expensive—yet effective—antimicrobial agent. This helps to preserve the efficacious effects of more powerful antimicrobial agents for future use. Antimicrobial susceptibility information obtained from an assay in accordance with the disclosure of U.S. Pat. No. 7,262,021 can be shown to be substantially identical to antimicrobial susceptibility information obtained using recognized testing methodology, specifically a Kirby-Bauer standardized antimicrobial susceptibility testing disk diffusion method that utilizes standardized full-size antimicrobial susceptibility test disks.
The user of the above-mentioned method places antimicrobial papers into one or several chambers containing growth media, as well as an organism sample spread thereon so as to facilitate growing an organism lawn. This placement can be done by individually placing the papers into the corners and abutting them against one or more walls along the edge of a chamber of the plate in a specific order. Following the incubation period, the user reads the results using a measuring device (such as a micrometer) to give a distance value in millimeters, as measured from the edge of the chamber at an antimicrobial agent sample location to a margin of a zone of inhibition. This value is then compared with a distance value on a zone radius interpretive table or chart to determine a result as to each agent. The result is given conventionally in terms of “resistant”, “intermediate”, or “susceptible”. The radius distance value generally speaking can usually also be doubled and compared to a standard Kirby-Bauer zone diameter interpretive chart to obtain an accurate test result.
The disclosure of U.S. Pat. No. 7,262,021 is hereby incorporated into this disclosure by reference for the disclosure therein consistent with the present disclosure. To the extent (if any) that it is inconsistent with the present disclosure, it is superseded for purposes of the present disclosure by the teachings herein set forth, and the present disclosure set forth shall control in the case of such inconsistency.
Standard Kirby-Bauer assays, and the inventor's prior assays just mentioned, require the just-described measurement, and interpretative table look-up, for each antimicrobial agent sample used in the test. This means time and care are required to accurately measure the distance, and to correctly use the table; so that an accurate test result can be obtained with regard to each agent tested. This adds to the time required for completion of the assay, and to the difficulty of performing it.